The light radiation theory of light-emitting diode (LED) is when a suitable voltage is applied to the LED, electrons are able to recombine with holes within the LED, releasing energy in the form of photons. Because the light radiation theory of LEDs is different from conventional light source such as the incandescent light which is through the heating of filament, the LED is also called a “cold” light source. Moreover, LED has the advantages of good environment tolerance, a long service life, and low power consumption so it is regarded as another option to replace the conventional light source. Particularly, the application of the UV LED has been used in UV curing, medical phototherapy, counterfeit detection, disinfection/purification, and light sources of phosphor-conversion white LEDs.
An LED has a light-emitting stack which is epitaxially grown on a growth substrate. The growth substrate can be removed via a laser lift-off process and replaced by a permanent substrate via a bonding process to form a bonding-type LED. The permanent substrate has a higher heat dissipating efficiency than the growth substrate. The laser lift-off process is accomplished by irradiating the light-emitting stack through the growth substrate with a laser. However, if the power of the laser is not controlled under an optimized condition while performing substrate removing, the laser beam may damage the light-emitting stack of the LED.